(1) Field of the Invention
The present invention relates to a heat-resistant adhesive and an adhesion method using the adhesive. More particularly, the present invention relates to a heat-resistant adhesive with excellent adhesion strength and an adhesion method using the adhesive.
(2) Prior Art
Heat-resistant materials such as carbon materials, ceramics and the like show no melting point or have a very high melting point, so that they are used under various high temperature conditions.
In processing these heat-resistant materials into molded articles, particularly those of large size or complicated shape, there have been pointed out problems such as necessity of large furnace, necessity of mold or post-treatment and consequent high production cost. Therefore, if it is possible to produce necessary parts of a desired molded article beforehand and assemble the parts with an adhesive into the desired molded article, the production steps of the molded article will be simplified and its production cost will be reduced.
Meanwhile, conventional organic compound adhesives are usable at a temperature range of 200-300xc2x0 C. and accordingly are unsuited for adhesion of heat-resistant materials used under high temperature conditions. This has posed a limitation to the applications of carbon materials.
As a heat-resistant adhesive, there is also known, for example, an adhesive obtained by solidifying a ceramic powder with a binder resin. This adhesive, however, has a low adhesion strength and moreover is unusable for adhesion of flat-surfaced materials.
The present invention has been made in order to solve the above-mentioned problems of the prior art and provide a heat-resistant adhesive with excellent adhesion strength and an adhesion method using the adhesive.
In order to achieve the above object, the present invention provides a heat-resistant adhesive comprising an easily carbonizable resin as a main component.
In order to achieve the above object, the present invention further provides a heat-resistant adhesive comprising an easily carbonizable resin and a powdery filler as main components.
In order to achieve the above object, the present invention furthermore provides a heat-resistant adhesive comprising an easily carbonizable resin, a powdery filler and a pitch as main components.
In order to achieve the above object, the present invention moreover provides an adhesion method comprising placing one of the above-mentioned heat-resistant adhesives between two materials to be adhered to each other and then subjecting them to a heat treatment to adhere the two materials to each other.
The present inventors made a study in order to solve the above problems of the prior art. In the study, the present inventors paid special attention to resins (e.g. polycarbodiimide resin) carbonizable by firing in a vacuum or in an inert atmosphere, and found that when an easily carbonizable resin(e.g. polycarbodiimide resin) was placed between heat-resistant materials (as mentioned above) to be adhered and they were fired at a temperature of 350xc2x0 C. or more, the heat-resistant materials could be well adhered to each other. This finding has led to the completion of the first adhesive of the present invention.
The present inventors made a further study to provide an adhesive with improved adhesion strength, capable of adhering even heat-resistant materials having a polished surface, strongly to each other. As a result, the present inventors found that an adhesive obtained by mixing an easily carbonizable resin (e.g. polycarbodiimide resin) with a powdery filler (e.g. graphite powder, carbon powder, coke powder etc.) gave good results. The finding has led to the completion of the second adhesive of the present invention.
The present inventors a study to achieve an even higher adhesion strength and, as a result, found that an adhesive obtained by mixing a polycarbodiimide resin with a powdery filler (e.g. graphite powder, carbon powder, coke powder etc.) and a pitch gave good results. The finding has led to the completion of the third adhesive of the present invention.
The present invention is hereinafter described in detail.
The easily carbonizable resin used in the present invention refers to, as explained above, a resin carbonizable by firing in vacuum or in an inert atmosphere, such as polycarbodiimide resin.
The polycarbodiimide resin used in the present invention can be a known polycarbodiimide resin or a polycarbodiimide resin which can be produced in the same manner as for known polycarbodiimide resins [reference is made to U.S. Pat. No. 2,941,966; Japanese Patent Publication No.33297/1972; J. Org. Chem., 28, 2069-2075 (1963); Chemical Review, 1981, Vol. 81, No. 4, 619-621; etc.]. It can be easily produced by subjecting an organic diisocyanate to a condensation reaction wherein the elimination of carbon dioxide takes place.
The organic diisocyanate used in the production of a polycarbodiimide resin can be any of aliphatic type, alicyclic type, aromatic type, aromatic-aliphatic type, etc. They can be used alone or in combination of two or more (the latter case gives a copolymer).
The polycarbodiimide resin used in the process of the present invention includes a homopolymer or a copolymer both having at least one repeating unit represented by the formula
xe2x80x94Rxe2x80x94Nxe2x95x90Cxe2x95x90Nxe2x80x94
wherein R represents an organic diisocyanate residue.
As the R (organic diisocyanate residue) in the formula, there are particularly preferred an aromatic diisocyanate residue [In the present specification, the xe2x80x9corganic diisocyanate residuexe2x80x9d refers to a portion remaining after subtracting two isocyanate (NCO) groups from an organic diisocyanate molecule.].
The polycarbodiimidization catalyst has no particular restriction and can be illustrated by conventionally used phosphorene oxides such as 1-phenyl-2-phosphorene-1-oxide, 3-methyl-2-phosphorene-1-oxide, 1-ethyl-3-methyl-2-phosphorene-1-oxide, 1-ethyl-2-phosphorene-1-oxide and 3-phosphorene isomers thereof or the like.
Specific examples of the polycarbodiimide resin include the following. 
In the above formulas, n is a degree of polymerization and is in the range of 10-10,000, preferably in the range of 50-5,000.
Incidentally, the terminal(s) of the polycarbodiimide resin may be blocked with a monoisocyanate or the like, and the polycarbodiimide resin described above can be obtained in a solution form, or as a powder precipitated from the solution.
The first adhesive of the present invention comprises, as a main component, an easily carbonizable resin such as the above-obtained polycarbodiimide resin. It can be processed into desired forms such as sheet, fiber and the like, in the same manner as employed in ordinary polymer molding. The obtained fiber can further be processed into a paper, a cloth, etc.
The second adhesive of the present invention is obtained by mixing an easily carbonizable resin such as the above-obtained polycarbodiimide resin with a powdery filler.
As the powdery filler, there can be mentioned, for example, a graphite powder, a carbon powder and a coke powder etc. The particle diameters of these powdery fillers are preferably 0.01-100 xcexcm, more preferably 0.1-40 xcexcm.
As the graphite powder, there can be used synthetic graphite, natural graphite, etc. As the carbon powder, there can be used carbon black, carbon obtained by thermal cracking, a powder obtained by carbonization of coke, etc. As the coke, there can be used petroleum coke, coal coke, etc. That is, the powdery filler usable in the present invention is a filler which can be used as a material for synthetic graphite.
The third adhesive of the present invention is obtained by mixing an easily carbonizable resin such as the above-obtained polycarbodiimide resin with a powdery filler and a pitch.
As the pitch usable in the present invention, there can be mentioned, for example, a binder pitch, an impregnating pitch and an asphalt pitch.
In the second adhesive of the present invention, the proportion of the powdery filler can be, for example, 100 to 5 parts by weight, preferably 90 to 10 parts by weight per 100 parts by weight of the easily carbonizable resin. In the third adhesive of the present invention, the proportions of the powdery filler and the pitch can be, for example, 80-5 parts by weight, preferably 60-10 parts by weight and 50-5 parts by weight, preferably 40-10 parts by weight, respectively, per 100 parts by weight of the easily carbonizable resin.
Similarly to the first adhesive of the present invention, the second and third adhesives of the present invention can also be used in desired forms such as solution, sheet, powder, paper and the like.
In effecting adhesion using the thus obtained heat-resistant adhesive, firstly the adhesive of the present invention is interposed between materials to be adhered, by coating on the material or by putting it between them. The materials to be adhered can be any materials such as carbon materials, ceramic materials, glass materials, other heat-resistant materials and the like.
Then, the materials having the adhesive of the present invention between them are subjected to a heat treatment to fire and carbonize the adhesive, whereby the materials are adhered to each other. The heat treatment is effected at a temperature of, for example, 350xc2x0 C. or more in a non-oxidizing atmosphere such as vacuum, inert gas or the like. The temperature of the heat treatment can be determined depending upon the heat-resistant temperature of the materials to be adhered, but is preferably in a range of 350-3,500xc2x0 C.
Before the above-mentioned heat treatment, it is possible to conduct a pre-heating, for example, at 60-300xc2x0 C. for 10 minutes 4 hours in order to stiffen the adhesive of the present invention.
The heat-resistant adhesive of the present invention comprises an easily carbonizable resin such as a polycarbodiimide resin and, as necessary, a powdery filler (e.g. graphite powder, carbon powder, coke powder etc.) and a pitch, so that the adhesive can strongly adhere heat-resistant materials and even materials having a polished surface.